TAD
by Zathara001
Summary: When Gibbs, DiNozzo, and McGee are sent on TAD to investigate a death at Cheyenne Mountain, they find more than they expected.
1. Chapter 1

Continuity note: This takes place in the middle of season 6 of NCIS and season 1 of Stargate SG-1, and the two are assumed to be roughly concurrent. It's a major disconnect in continuity, but that's where the story feels like it fits in my mind.

As always, all rights in this work are given to those who own NCIS and Stargate SG-1.

CHAPTER ONE

Leroy Jethro Gibbs stood beside his director, Leon Vance, before the big screen in MTAC. It was always a pleasure to speak with his former boss, Tom Morrow, now a director at Homeland Security, but when speaking with Morrow discomfited Vance, the pleasure was doubled.

"Did I understand you correctly, sir?" Vance asked, clearly disturbed. "You want to borrow NCIS's best team for an assignment of undetermined length at an undisclosed location?"

Morrow considered that for a brief moment. "Not entirely accurate. I am assigning Agents Gibbs, DiNozzo, and McGee to temporary duty at an Air Force installation in Colorado."

"Not Officer David?" Vance asked, and Jethro concealed his irritation at the question. There was a lot to respect about Tom Morrow, and not least was his precision.

"No," Morrow said. "Even if she weren't Mossad, she's not a U.S. citizen. Jethro - your team will report to Colonel Kennedy at Joint Base Anacostia-Bolling to be read into the assignment and receive further orders."

"When?" Jethro asked.

"An hour."

"We'll be there, sir," he said, and barely nodded to Vance before turning to head back to the bullpen.

"Appreciate you not making me call SecNav or the President, Leon." Morrow's voice faded behind him, and Jethro wondered what the foxtrot they were getting involved in now.

He loped down the stairs and toward his desk. "DiNozzo, McGee - with me."

The other two grabbed guns and gear, just as he did, and started toward the elevator - Ziva close behind.

"Not this time, Ziva," Jethro said, and she turned a frown his way.

"But -" she began.

He cut her off. "Orders."

He could tell Ziva wanted to protest, but she sat back down, scowling.

They stepped into the elevator, and Jethro waited for the doors to close before saying, "Be at Anacostia main gate in fifty minutes. Pack for a week in Colorado."

He didn't even have to turn to know that DiNozzo was grinning, and he added, "Doubt we'll have time for skiing, DiNozzo."

NCIS - SG1 - NCIS

At Anacostia, they were escorted by an airman to a windowless conference room and told to wait there. Jethro nodded acknowledgment and looked around for a coffeepot. Maybe Air Force coffee would be better than Navy coffee, but he doubted it.

"What's going on?" McGee asked.

Jethro looked at DiNozzo, who grinned slightly. "C'mon, Probie - you're supposed to be an investigator. What do you think is going on?"

"Well - uh - I think we're being sent somewhere, but where and why?"

"Pretty sure they're going to brief us on that," DiNozzo said, and took a seat at the conference table, pulling out his cell phone to tap at its keys.

"But -" McGee looked flustered, and after a sip of coffee that actually was slightly better than Navy coffee, Jethro took pity on him - a little.

"We'll know what we need to know when we need to know it," he said, and McGee nodded, once, though he still looked uncomfortable.

 _When they needed to know it_ turned out to be less than five minutes later, when the conference room door opened and a man in a colonel's uniform strode in.

"Gentlemen," he said. "Thank you for coming."

"Our pleasure," Jethro replied and glanced at the man's nametag. "Colonel Kennedy."

"Time is of the essence, so I'll be brief," Kennedy said. "You are going to a base everyone knows exists."

"Cheyenne Mountain," DiNozzo murmured, his cell phone face down on the table beside him. Jethro bit back a chuckle as McGee stared dumbfounded at the senior agent. DiNozzo just grinned.

"Yes," Kennedy confirmed. "Some things that go on there are _black_."

The inflection made Jethro's eyebrows lift. "More than NORAD black?"

Kennedy nodded. "You will not mention anything you see or hear there outside the _black_ part of the base, even to personnel stationed at the complex, as not all of them are read in on the _black_ project. You only know this much because you very likely will see _black_ parts of the base as you do your job."

"What about Director Vance?" McGee asked.

"What about him?" Kennedy countered.

"What do we tell him?"

"He knows what he needs to know," Kennedy said. "He doesn't need to know anything else."

"Why us?" DiNozzo asked.

"Director Morrow has the utmost faith in you. I hope it's not misplaced."

DiNozzo shook his head. "I meant, why NCIS? Shouldn't AFOSI be investigating whatever happened?"

"Normally, yes," Kennedy replied. "But her death is one of the things you're investigating."

"Whoever killed her might come after us," Jethro observed.

"It's possible."

Jethro grinned. He liked a challenge. "Hoo rah."

NCIS - SG1 - NCIS

They took a sleek executive jet to Peterson Air Force Base, and Tony managed not to gush over its amenities, before they were driven to the Cheyenne Mountain Complex, passing several security checkpoints before finally pulling into an open hangar-like space. Their driver parked and escorted them to the access checkpoint, where a staff sergeant and a couple of MPs stood duty.

"Agents Gibbs, DiNozzo, and McGee," their driver announced.

The sergeant - whose name, Tony noted automatically, was Lawrence - greeted them with, "You'll need to secure all electronic gear - cell phones, laptops, smart watches - and firearms in the lockers before you go any further."

Gibbs paused, his cell phone already halfway out of his pocket. "We're not surrendering our weapons."

Sergeant Lawrence leveled a glare at Gibbs, and Tony could only hope he hid his smile at the effort. It wasn't even half as menacing as Gibbs' mildest glare. "It's part of the procedure, sir."

"There's an exception for federal law enforcement officers," Gibbs replied calmly, not in the least fazed by the man's attempt at intimidation. "Your CO can confirm that, Sergeant."

Tony didn't hear any extra inflection on the man's rank, but it was nonetheless a reminder that NCIS was here to do a job and they wouldn't be hampered.

Lawrence gestured to the desk. "Your bags, please. We can search them while the weapons issue is resolved."

It was a reasonable request, and while the sergeant picked up a phone and spoke quietly into it, Tony followed Gibbs' lead, hefting his suitcase and gear bag onto the security desk. Those four bags took up most of the desk space, and Tony stepped back while the MPs began their search of the luggage.

It was only because he'd taken those steps back that Tony saw a door at the far end of the room opening - an armored door, he noted even as he studied the man who came through it. The newcomer had medium brown hair and walked with an easy confidence that emphasized his height, taller than Gibbs but shorter than Tony himself, Tony decided as the man drew closer.

"What's the problem, Sergeant?" the man asked.

"They're refusing to surrender their firearms."

"Sounds like you need to pay more attention in your security training," the man - a colonel, Tony realized now that the eagles embroidered in blue on the shoulders of his ABU were visible. "They're federal law enforcement officers. There's an exception."

Sergeant Lawrence nodded, appearing only somewhat abashed. "Yes, sir."

"I reminded him," Gibbs observed.

"Never gonna blame a man for double-checking." The colonel held out his hand. "Jack O'Neill. Two Ls."

Gibbs took his hand. "Jethro Gibbs. Two Bs. Agents DiNozzo and McGee."

"You're clear," Lawrence said, and Tony stepped forward to retrieve his bags. Beside him, McGee hefted two suitcases and his gear bag onto the desk.

"Got your electronics secured?" O'Neill asked.

"Finishing up now," Tony answered.

A low whistle from the desk made O'Neill, Tony, and Gibbs turn. One of McGee's suitcases had been opened, and Tony wouldn't have expected it could hold the amount of electronic equipment currently on display.

"None of this goes any further," Sergeant Lawrence said. "The clothing you've used to cushion the equipment can go in the other bag, but the gear will have to be secured."

Gibbs took a step closer to get a better look, then scowled at McGee. "What's all this, McGee?"

"It's a game console," McGee said. "I always take one with me. Playing the game helps me relax."

"It's an electronic device," Sergeant Lawrence said. "It has to be secured with all the others."

"But -" McGee began.

"Into the locker," O'Neill said, and though his tone was polite, it brooked no argument.

McGee appeared to sigh silently, but stepped forward to close the suitcase.

"What's that on your wrist?" O'Neill asked, and Tony strained to get a better view.

"It's a fitness tracker," McGee said.

"Electronic device. Into the locker," O'Neill ordered then directed a baleful look toward Tony and Gibbs. "What about you two?"

Tony grinned. "Cell phone's in the locker already."

O'Neill's eyebrows shot up. "Just a cell phone?"

Gibbs shrugged. "We know it's a black op. Why would we bring anything else?"

Tony saw McGee flush as the younger agent finished securing his gear - cell phone, laptop, game console, fitness tracker - before turning back to them, but couldn't find it in himself to feel badly for the younger man. Surely he'd learned about security procedures for classified installations at FLETC, and if not there, then in his years of service at NCIS. If he hadn't, or if he hadn't been paying attention, he'd brought this on himself.

"Ready?" O'Neill asked, and Tony couldn't help but be impressed when there was only professional courtesy in the colonel's tone. If Gibbs, or even he, had asked the question, it would've been loaded with sarcasm.

Gibbs glanced over and Tony nodded. From the corner of his eye, he saw McGee nod, too. Gibbs said only, "Ready."

Tony trailed behind as they followed O'Neill to the door he'd come from. Tony wasn't surprised when O'Neill had not only to swipe a security card but also enter a code - nor when O'Neill angled his body so the numbers he pressed weren't visible - before the door opened. The door led to another corridor, then another door, and an elevator.

"I'm your liaison officer while you're here," O'Neill said when the doors to the elevator closed.

"A colonel as liaison officer?" Gibbs asked. "Seems like something a lieutenant would be assigned to."

"Yeah, well, we do things a little differently here," O'Neill replied.

"Including bringing NCIS to an Air Force investigation," Tony murmured.

"You're catching on," was all O'Neill said in response.

Anything else Tony might have said was cut off when the elevator doors opened onto what looked like a bunker out of some old 1970s movie - but then, Tony remembered that the complex had been built in the mid-60s, and it was probably advanced for its time, let alone whatever upgrades had been done in intervening decades.

They were silent while O'Neill escorted them to their quarters - not much bigger than Tony had had while agent afloat aboard the _Ronald Reagan_ and the _Seahawk_ , but they all had individual billets and didn't have to hot rack, so Tony wasn't complaining.

Then O'Neill escorted them to a conference room on the same level as their quarters.

"Your base while you're here," O'Neill said, and Tony followed Gibbs into the room and took a seat beside him, McGee on his other side.

"How about giving us the full briefing?" Gibbs asked.

O'Neill smirked. "The briefing you need now, at least." Then he sobered. "Four days ago, Major Lou Thomas turned up dead."

"Cause?" Gibbs asked.

"Still under investigation," O'Neill replied. "No obvious injuries and tox screen came back clean, but he was one of our team commanders, so we had OSI looking into it."

"Colonel Kennedy said the OSI agent's also dead?" Tony prompted.

"Yesterday," O'Neill said. "Her throat was cut."

"That the official cause of death?" Gibbs asked.

"So far," O'Neill replied. "The autopsy's complete, but we're still waiting for the tox screen to come back."

"Weapon?" Tony asked.

"Lying on the floor beside her."

"Fingerprints?" McGee asked, and Tony thought he sounded hopeful, almost eager.

"We sent it to the Springs for analysis," O'Neill said. "We don't have a full forensics lab here."

"So our job is to figure out who killed Major Thomas and the OSI investigator…?" Tony let the question trail off.

"Major Madison Morgan," O'Neill supplied.

"And find out if there's a connection between them," Gibbs finished for Tony. "McGee."

McGee straightened in his seat, and Tony thought he looked glad to have a task assigned to him. "If you'll show me to a computer, I can download my search algorithms and get started."

"Download?" O'Neill frowned. "From where?"

"I have a secure site in the cloud," McGee replied. "I keep all my programs there."

O'Neill shook his head. "I meant, from what computer? None of ours are connected to the Internet."

"Well, that's why I brought my laptop," McGee said. "I can use it -"

"McGee," Gibbs snapped. "You do remember we're at a black site, don't you?"

"Yes, Boss," McGee said, apparently puzzled, and Tony struggled to contain his smile.

"Your laptop isn't black," Gibbs growled. Then he glanced at Tony, and Tony took his cue - if O'Neill were liaison officer for the Air Force, then he was liaison officer for NCIS.

"I assume you have personnel files here, Colonel?" Tony asked. At O'Neill's nod, he said, "Paper or computer?"

"Bit of both," O'Neill replied.

"Can McGee have access to those?"

O'Neill considered that for a moment, then nodded. "Not a problem."

Tony smiled at the colonel. "Thanks. If you can have Major Morgan's case file on Major Thomas brought in, I'll get started."

"I want to see where both victims died," Gibbs said. "DiNozzo's taking lead on Thomas's death. I'm taking lead on Major Morgan's death."

"I'll need to talk to the other members of Major Thomas's team," Tony added, and held up a hand to forestall O'Neill's protest. "I won't ask about operational details, just behavior and relationships."

"Same here, with regard to Major Morgan's friends and coworkers," Gibbs said.

"She was OSI, Boss," Tony reminded him. "Probably didn't have a lot of friends here." He certainly hadn't had a lot of friends when he was afloat.

O'Neill surveyed them for a moment, and Tony saw a grudging respect in his expression. "I'll have the files brought in, then I'll take you to where they died. Captain Carter will escort Agent McGee to a workstation."


	2. Chapter 2

After being introduced to Captain Doctor Samantha Carter, who O'Neill indicated was their secondary liaison officer, Jethro followed O'Neill out of the conference room.

"Your agent have a problem keeping secrets?" O'Neill asked as they waited for the elevator.

Forcing thoughts of _Deep Six_ and its sister novels aside, Jethro shook his head. "He's just part of the Internet generation. Everything's available on the web - YouTube, Facebook, Twitter, Instagram, Pinterest, that sort of thing."

O'Neill stepped into the elevator and regarded Jethro seriously. "No idea what any of those are."

Jethro chuckled. "Me, either, but McGee, DiNozzo, and Abby talk about them all the time."

O'Neill chuckled with him, and the elevator doors opened onto another corridor that looked nearly identical to the one they'd left. Jethro supposed that, like an aircraft carrier, there were designations to identify which level, section, and corridor they were in, but he hadn't yet seen enough of the base to even begin to sort those out.

After several turns, O'Neill paused in the middle of a corridor. "Thomas collapsed right here."

Given that Thomas had been found several days before, Jethro wasn't surprised the corridor was clean. A blacklight might find traces of blood or other bodily fluids, but Jethro doubted it given the hard floors and the type of cleaning solutions he expected were used in a place like this.

Still, Jethro examined the floor and walls closely, careful not to get too close to any doors - he had no need to know what might lay behind them, let alone run the risk of being slammed into if they opened too quickly. As he expected, the area was clean.

He pointed at an alphanumeric designation painted on the wall opposite him, and asked, "Anything special about right here?"

O'Neill considered the question, and Jethro couldn't help glaring at him. "We can't do our jobs if we don't have the information we need. Talk in generalities if you have to, but give us something."

O'Neill studied him for a moment. "You're familiar with black ops."

"I'm a Marine sniper," Jethro replied. "I've done my share of black ops. Don't say anything you shouldn't, but say what you can. Assuming you want justice for Majors Thomas and Morgan."

O'Neill nodded, once. He wasn't apparently offended by Jethro's last barb, and Jethro thought he might have found an officer he could like - a little - as well as respect. "Generalities."

It was another moment before O'Neill composed his thoughts enough to continue. When he did, he pointed past Jethro. "Operational ready room behind those doors. Thomas and his team came out after mission debrief, and he collapsed here."

"Who performed the autopsy?"

"Dr. Warner, our chief medical officer," O'Neill replied.

Jethro caught the man's title. "Not a medical examiner?"

"No."

"Will you let our ME looked over the autopsy results?" Jethro asked. Normally, he'd insist that Ducky be allowed access, but he knew enough of black ops to know that he had no authority to insist on anything here.

"Who's your ME?" O'Neill asked.

"Ducky - Donald Mallard," Jethro said. "Former SAS."

O'Neill's eyebrows rose. "I'll talk to the general, ask him to approve it. If so, we'll have the records sent to him ASAP."

"Thanks, Colonel." His team, Jethro reflected, would probably be surprised by how polite he was being, but this wasn't his turf. Not to mention that old military habits died slowly and were quick to return in certain circumstances.

He surveyed the area again, just to be sure. Then, "Anything unusual in the ready room? Thomas behave differently? Say anything odd?"

"I wasn't here at the time," O'Neill said. "But I'll tell Thomas's team to answer those questions."

"DiNozzo will be asking them," Jethro said absently. "He's lead on Thomas's death." With a last look around, Jethro met O'Neill's gaze once again. "Where did Major Morgan die?"

"This way."

NCIS - SG1 - NCIS

The moment Tim McGee laid eyes on Captain Carter, he had to stifle a groan. There was no way Tony wouldn't flirt with the blonde, blue-eyed woman who'd come to the conference room in response to O'Neill's call carrying an armload of files, and Tim had had enough of Tony's flirting to last a lifetime. It just reminded him how unlucky he usually was with women.

"The files you requested, Colonel," she said.

"Captain Doctor Samantha Carter," O'Neill said, then gestured to Tim and his companions in turn. "Agents Jethro Gibbs, Tony DiNozzo, Tim McGee, of NCIS."

Each man nodded in turn, and O'Neill continued, "McGee needs a workstation."

The captain glanced at Tim before looking at the colonel once more. "Access?"

"HR files, plus Thomas and Morgan's LESs and personal information," O'Neill said. "Anything else, check with me or Hammond."

The captain barely had time to acknowledge the order before O'Neill turned to Gibbs. "This way, Agent Gibbs."

With a polite, "Ma'am," Gibbs followed O'Neill from the room, and Tim waited for the flirting to begin.

To his surprise, Tony simply took the files from Captain Carter. "Do you prefer Captain or Doctor?"

She blinked once, then gave a small smile. "Captain."

"Pleasure to meet you, Captain." Tony didn't even smile, and Tim wondered if his fellow agent was feeling all right. When didn't Tony flirt?

"You understand you need to be escorted if you leave this room?" she asked.

Now Tony did smile. "Pretty sure the sergeant outside will make sure I stay here, or go with me if I need to leave."

Tim flicked a startled glance at the door and, yes, an armed sergeant stood outside the door. When had he arrived and why hadn't Tim noticed?

Now Tim hurried to keep up with Captain Carter as she strode through one gray corridor after another. He still couldn't believe Tony hadn't flirted with her, but Tim wouldn't pass up the chance and sought a good topic of conversation.

"What's your doctorate in?"

Her pace slowed just a little, and when she looked at him, it was almost a challenge. "Theoretical physics."

Tim cheered inwardly - there was no way someone with a doctorate in theoretical physics would be interested in a guy like Tony.

What he said aloud was, "Cool. Where from?"

"MIT."

"Really?" Now Tim grinned. "That's where I got my masters. Computer Forensics. I also have a biomedical engineering degree from Johns Hopkins."

Captain Carter - he wanted to think of her as Samantha - gave him an assessing look, but all she said was, "Interesting."

Tim floundered for something to say in response, but before he found it, she led him into a small, almost cramped, office with two computer workstations set up. Two very large, very antiquated-looking computer workstations, Tim corrected with a silent groan. The keyboards were putty colored, for crying out loud.

"All set, Lieutenant?" Captain - Samantha - asked.

The dark-skinned man sitting at the nearer workstation looked up at her question. "Yes, ma'am, per the colonel's instructions."

"Agent McGee, Lieutenant Stovall. He'll answer any questions you have."

"Er - thank you, Captain," Tim said. "I'll see you later."

But she was already gone. Tim shoved disappointment down deep - he'd hoped Samantha would be the one to help him - and took a seat at the empty workstation.

Then he saw the screen in front of him and almost groaned aloud when he saw a green rectangular cursor blinking against a black background.

"You guys are on _DOS_?"

Lieutenant Stovall shrugged. "It does what we need it to."

"This is gonna take forever," Tim muttered, but set to work.

NCIS - SG1 - NCIS

Tony knew, understood, and accepted all the ramifications of being read in on a project like - well, like whatever this one that had brought him to an Air Force installation in Colorado was. Even so, he felt the tiniest tremor of nerves as he reached for the file on Major Lou Thomas.

Of course it was redacted, but not nearly as much as he'd expected. The only things that had been blacked out had to do with this project, so Tony skimmed over previous postings - Thomas had been a front-line guy in Afghanistan and Iraq - and the medals he'd earned - Air Force Distinguished Service Medal, Purple Heart, Air Force Combat Commendation Medal, and a handful of campaign medals. Major Thomas had been dedicated to his career, and his country, if his file were anything to go by.

"Sir."

Tony looked up to see a lieutenant in BDU standing at attention in the doorway. The man had sandy hair and … freckles. Tony hoped he hadn't been hazed too badly.

Tony checked the man's name and waved him inside. "Have a seat, Lieutenant Calloway."

Calloway sat and Tony glanced at another file he'd been given to refresh his memory. "You were Major Thomas's second in command?"

"Yes, sir."

"Tell me about him." Tony leaned back in his seat and gave his attention to the other man. From this angle, Tony saw the unit patch on Calloway's right shoulder. The colors were muted enough that he couldn't make out details, but he saw the letters SG and the number 5 clearly enough.

"Good man, good commander," Calloway said, and that seemed to be that.

"You hang out with him off duty?" Tony asked.

"Sometimes - he'd take us out for a beer on our birthdays, that kind of thing." Calloway met Tony's gaze steadily. "Why not ask what you really want to know?"

Well, with an opening like that, there was only one question Tony could ask. "Did you kill him?"

" _No._ " Calloway looked horrified at the question - or maybe the implications, because his next words were, "Do you think somebody did?"

"I think he collapsed after mission debrief, and nobody knows why," Tony said. He sat forward. "NCIS investigates all suicides and questionable deaths as if they were homicides. OSI does the same thing, or else why was Major Morgan looking into his death?"

"Because we're -" Calloway broke off, and Tony could see his expression closing down around security concerns.

"Part of a black ops unit," Tony finished. "I get it. I'm just saying, if it were natural causes, they'd already know. Could it have anything to do with your last mission?"

Calloway's expression closed off even further. "Why do you ask?"

"Because he collapsed right outside your -" Tony checked his notes. "Colonel O'Neill called it the operational ready room. Tell me what you saw, heard."

Calloway considered that for a long moment, then said, "We'd just gotten back, and we were heading out for showers and to hit the sack. I was behind him, and he'd barely cleared the door before he … fell. Not a word, not a gesture, just fell. I was closest, so I went to help - first aid, CPR, whatever I could do - and someone, maybe Kingston, yelled for a medic. But he was gone already. We couldn't bring him back."

Calloway swallowed at the end, and Tony gentled his tone for his next question. "So your team is Major Thomas, you, and Kingston?"

Calloway nodded. "And Doctor Brown."

"Was that routine? Get back, head for showers and the sack?" Tony asked.

"Yeah, as much as anything's routine here."

"Everyone on the team get along?"

"Just like any group, we have our quirks, but yeah, we get along. Nobody had a grudge against the major that I know of."

"Would you?"

To his credit, Calloway thought about the question. "If it was here, yeah - have to, to watch his six, you know?"

Tony grinned. "I know."

"Personally - I'm not sure I would."

"What about Doctor Brown?"

"Brown?" Calloway shrugged. "Typical egghead. Gets lost in the clouds sometimes, you know?"

"The Air Force does spend a lot of time in the clouds," Tony quipped, and Calloway chuckled, then looked surprised that he'd done it. Good - the man was relaxing a little. Maybe Tony could finally start getting answers.

"Everyone acted normally on your last few missions?" Tony deliberately made the question more inclusive, hoping it might jog something loose in Calloway's memory. "Any changes that you noticed?"

"No, sir. They've been fairly routine - as routine as anything gets around here, anyway."

"You were together the whole time you were away? Anyone go off alone?" Tony prompted.

"We stick together," Calloway said. "'Cept for taking a dump, and even then we're within earshot."

So it was possible - however unlikely - that something had happened to Thomas on mission and his team hadn't known. Tony filed that tidbit away.

"And on the way back from Peterson?"

"Peterson?" Calloway blinked, and for a moment Tony thought the other man didn't know what he was talking about. Then Calloway resumed his normal expression. "Nothing unusual, sir."

Tony noted the hesitation, but had no idea what it meant. "I think that's all for now, Lieutenant."

"Sir." Calloway stood, straightened to attention for just a moment, then left the room, leaving Tony wondering just what was so unusual about referring to the drive back from Peterson.

Whatever it was, he was sure he'd figure it out - after he interviewed Doctor Brown.


	3. Chapter 3

The site of Major Morgan's death provided a little more information than that of Major Thomas's death, Jethro noted, if only because the blood hadn't been cleaned away yet.

"Goes against every instinct I have to leave it like that," O'Neill said from where he lounged against the opposite wall. "But Director Morrow said you needed as much information as you can get."

Jethro felt an unwilling grin tug at his mouth. "He's right."

"Let me know when we can clean it up."

"Roger that." Jethro studied the scene. He knew from the reports he'd already read that Morgan's throat had been cut, but the scene itself suggested she hadn't fought back. Obviously, he'd wait for confirmation from the autopsy report, but a dozen years as an investigator had taught him a lot about reading a crime scene, and he was confident the report would confirm his observation.

"She was investigating Major Thomas's death," Jethro said. "Did she have any other active cases?"

"If she did, there wasn't anything major."

Jethro turned a glare on O'Neill. "Because you know everything she was working on?"

O'Neill returned his glare with a bland expression. "No. But I know the people here, on this project, and we're all dedicated to its success. Maybe she had a case or two upstairs, but she wasn't investigating anything here. You can ask General Hammond to confirm that if you want."

"I will." Jethro took a last look around. "You can clean this up. How about her office and her quarters?"

"This way."

Half an hour later, Jethro concluded his search of Major Morgan's office. Everything seemed in order, though he'd have McGee go through her files more closely to be sure.

"She have an office upstairs, too?" Jethro asked.

"Just this one. Simpler to keep her office in the more restricted part of the base - anything that goes on upstairs is less sensitive than what goes on here."

"Efficient."

"We try."

"Her quarters?"

"This way."

Jethro fell into step beside O'Neill. "Was she seeing anyone?"

O'Neill chuckled. "I'd be the last person to know if she was."

"Fraternization regs?"

"Lack of interest in gossip," O'Neill corrected. "Hard to keep secrets in a place this size, though, so if she was, someone will know."

"What do you know about her?" Jethro challenged.

"Aside from her name and job?" O'Neill shrugged. "I could pick her out of a lineup, but she wasn't in my command."

"What bits of gossip have you heard - everyone hears some." Jethro grinned to himself. Questioning O'Neill was a challenge. He couldn't deny the voice in his head saying, _Like questioning yourself, you mean._

"She was popular," O'Neill said. "Or at least friendly - said hi, seemed to know everyone and remember things she heard about them."

"Sounds like she was a good agent."

"Maybe. Like I said, not in my command."

It was interesting how quickly a challenge could turn from interesting to frustrating. "Workaholic, Colonel?"

"Jack," O'Neill corrected absently. "More that I was enjoying my retirement until I got called back up to active duty for this. I get home as much as I can."

"Where's home?"

"Cabin in the middle of nowhere." Which wasn't really an answer, Jethro thought, but it would have to do for now.

Jethro recognized Major Morgan's quarters immediately - an airman stood guard outside it. The man straightened to attention at their approach, and O'Neill gestured him to stand aside before taking a key from his pocket and unlocking the door.

"It locks?" Jethro questioned.

"Officer and OSI," O'Neill replied. "It locks."

"Anyone been in here since she died?"

"We've had a guard stationed outside since her body was found. It's possible someone got in and out before that."

Jethro nodded an acknowledgment and pulled on a pair of gloves as he began his search of the room.

NCIS - SG1 - NCIS

Tony was reviewing the statements of Major Thomas's team for the third time when the sergeant stationed outside the conference room said, "Doctor Brown, sir."

Tony didn't bother to look up, or to hide his chuckle. "First name Emmett?"

"Emily, actually."

Now Tony looked up, hoping he concealed his surprise - he hadn't looked over all the personnel records yet - at the middle-aged woman standing in the doorway.

Despite his surprise, he smirked. "You have a dog named Copernicus?"

Her mouth twitched. "No."

"Einstein?"

"No." She tried to sound severe, but her smile gave it away. Tony just quirked an eyebrow at her. "I named the _dog_ …"

"…Indiana," Tony finished with her, grinning widely. "Finally! Someone who appreciates movie references."

Doctor Emily Brown laughed. "Considering Indiana Jones got me interested in archaeology, it seemed only fitting."

Tony waved her to a seat. "He got a lot of people interested in archaeology. Not everyone followed through on it… as I assume you did?"

She sat in the same seat Calloway had taken, and now that she was closer, Tony could see the fine lines starting to appear at the corners of her blue eyes and the gray starting to show in her brown hair.

"A doctorate in archaeology and a second bachelor's in anthropology," she replied. "But unlike the esteemed Professor Jones, I try to avoid grave-robbing and ancient deadly traps."

Tony concealed his surprise - he hadn't expected her to have those particular degrees. But he could ask, "What's a friendly archaeologist like you doing in a military installation like this?"

"Consulting," she replied easily enough, and Tony grinned again - this time to himself.

Unlike Calloway, Dr. Emily Brown would be a challenge - cagey, aware of her responsibilities and surroundings, and not inclined to say anything outside the required responses to his questions.

Still, he had to try to get her chatting. "So that we don't inadvertently destroy something of historical or cultural significance?"

"Something like that," she replied.

Tony waited, but she said nothing more, and his respect for her inched up a notch.

"Did you notice anything unusual about your last couple of missions with Major Thomas?"

She laughed. "Oh, honey - around here, _unusual_ is a way of life."

"Anything more unusual than usual, then?" Tony suggested.

She appeared to consider the question, her lips pursed and her gaze focused somewhere past Tony's shoulder.

"No," she said finally. "Nothing I can recall."

"Not even a fancy inscription or pottery sherd to break up the monotony?"

Her eyes widened. "You said _sherd_."

Tony blinked. "Uh - yes. Why?"

"Most people say _shard_." She grinned at him, and the years and care etched in her face melted away.

Tony couldn't help grinning back. "What can I say? I took archaeology as one of my required humanities courses in college."

"Inspired by Indiana Jones?"

"What do you think?"

It was rare that he got to laugh during an investigation, and he certainly hadn't expected to laugh during this one, so Tony let the moment linger. Then he leaned forward.

"He was right, you know - artifacts belong in museums," Tony said. When she nodded, he added, "And criminals belong in jail. If there's anything you can think of that might help us put this one in jail, I need to know."

"I want to help, Agent DiNozzo," she said. "But really, it's been routine. Go out, come back, go through quarantine, write reports, and do it all again."

"Quarantine?" Tony repeated.

"To be sure we don't bring anything back with us, that's all."

"What, like Zika virus?"

"That or parasites," she answered. "We all came back clean, though - you don't have to worry."

"That's a relief," Tony murmured, even as he made another note on his pad. _Quarantine?_

He asked a few other questions - mostly variations on ones he'd already asked, just to try to shake something else, anything else, loose - but after a few more minutes, he thanked her for her time and cooperation.

"I just wish I could've been more help," she said.

"We do what we can, Doc," Tony said, and then grinned. "I can call you Doc Brown, can't I?"

She studied him for a moment. "Only because you said _sherd_ correctly."

NCIS - SG1 - NCIS

"Telephone call for you, Agent McGee," Lieutenant Stovall said.

"Thank God," Tim muttered, and then check to be sure Stovall hadn't heard it. If he had, the other man gave no sign, and Tim breathed a silent sigh of relief.

He knew that every aspect of an investigation was important, even the ones that weren't glamorous or that didn't lead directly to a suspect. He knew that, but he still felt halfway to useless being stuck in this windowless cubicle with a computer than ran on a decades-old system and as slowly as its age would suggest.

Thankfully, he could focus on something else for now. "Call?"

"A Doctor Mallard," Stovall said. "Putting him through to line two."

Tim grabbed the phone and stabbed the button for the proper line. "Ducky?"

"Ah, yes, Timothy, it is good to hear your voice. Are they treating you well at your undisclosed location?"

Tim couldn't help chuckling at Ducky's unflappable cheer. "So far, anyway. What's up?"

"I've reviewed the autopsy reports per Jethro's request," Ducky said, "and I agree with the conclusions. Major Thomas died of natural causes, and Major Morgan of exsanguination thanks to having her throat cut."

"That's it?" Tim knew he sounded forlorn and summoned some of his usual cheer to add, "You're usually the one to set us on the right track, after all."

"Thank you, Timothy. That's very kind, however untrue it may be. But as it happens, I do have one other tidbit to pass along."

"What?" Tim scrambled for the pen and notepad he'd been given. Tony and Gibbs had ones just like it - they could make all the notes about the case they wanted or needed to, and when they left, the notepads would be handed over to Colonel O'Neill.

"I'm not certain it has any significance, but I did note something unusual about Major Thomas's brain."

"What?" Tim tucked the handset under his chin and turned to a fresh page on his notepad.

"I thought I saw some dust on the photographs of the brain, so I sent them to Abby. She enlarged and enhanced them, and what I thought were specks of dust were in fact a pair of tiny holes in the base of the cerebellum," Ducky said. "And by tiny, I mean less than a millimeter in diameter."

"What does that mean in layman's terms, Ducky?" His biomedical engineering degree was of no help here, so he held his pen poised to record whatever Ducky said next.

"I have no idea," Ducky replied. "The brain is a single organ, and without knowing where the holes terminate, I can't begin to determine their meaning. Except…"

"Except?" Tim prompted.

"According to the latest research in neuroscience," Ducky said, "it would conceivably be possible - just _possible_ , you understand - that a single fiberoptic thread that touched every area of the brain could - again I stress _could_ \- influence every aspect of a person's brain and, by extension, body."

"I see," Tim said, though he didn't. The whole scenario sounded like something out of a horror movie, or a grade-B science fiction film, not the basis of a serious investigation.

"Oh, and do tell Jethro that Abigail concurs with whoever performed the toxicology analyses. Neither Major Thomas nor Major Morgan had any of the most common drugs in their systems."

"I'll do that, Ducky, thanks," Tim said. Not for the first time since he'd arrived, he wished he were back in Washington, back at the Navy Yard. At least there, he knew his purpose and place in the pecking order.

Here, he was little more than a glorified desk jockey, searching through records that were, as one would expect at such a classified installation, clean, even pristine.

Tim ended the call and, with another silent sigh, went back to his search.


	4. Chapter 4

"Agent DiNozzo?"

Tony looked up from the files he'd been reading - the classified, partly redacted personnel files of all of Major Thomas's team - and blinked his eyes into focus at the sergeant standing in the doorway.

"Yes?"

The sergeant smiled, just a hint of an expression. "Chow hall's about to close up for the night."

"Is it? Huh."

Tony rose from his chair and stretched the kinks out of muscles too long in one position before gesturing toward the sergeant. "After you."

Trusting the sergeant wouldn't lead him anywhere he wasn't supposed to be, Tony allowed himself to look around.

Not that there was much to see, except for the people - and Tony frowned when he realized that there weren't just Air Force uniforms present, but also some Marines. What the hell kind of command was this - some joint operation?

But if that were so, where were the Army and Navy? And why were there a handful of civilians, too?

 _Stop asking questions that can get you thrown in prison,_ Tony told himself. That he was only asking the questions in his mind was a relief - he just had to be sure not to ask them aloud.

But the questions lingered, and Tony found himself studying the people he passed as a distraction.

 _Male, scientist type, brown eyes behind wire-rimmed glasses, easily distracted. Not military despite the black T-shirt and olive drab trousers._ Tony dodged aside a step to avoid running into the other man, who didn't even look up. Ahead of him, the sergeant snickered.

 _Female, Asiatic origin, do not mess with her,_ his mind told him. _May look like five and a half feet of nothing, but carries herself like a fighter._

 _Male, dark-skinned, cool tat on his forehead. Not gang. Tribal?_ The question focused Tony's gaze on the tattoo, only it wasn't so much a tattoo as a scar. A deliberate scar, Tony corrected himself, of a curvy U shape inside two ovals, and tinted in gold.

The man's steady, serious eyes met his, and Tony forced himself to nod an apology, even as he noted the sergeant straightening his already military posture as they passed.

 _Something off about that guy - but what?_

The question occupied Tony's thoughts as he arrived at the chow hall, got dinner, and joined his teammates at a table on the far side of the room.

"Glad you could join us, Tony," McGee said, his lips curved in a smirk. Tony blinked and realized that he and Gibbs were almost done eating.

"Got caught up reading personnel files." Tony took his seat and dug in, only now realizing how hungry he'd let himself get.

"Find anything?" he asked around a mouthful of meatloaf.

Gibbs glared at him - probably for voicing what was usually his question, Tony thought, and grinned at his boss. Gibbs apparently decided to let it go, because he nodded at McGee.

"I didn't find anything suspicious in the personnel files I looked at," McGee began. "Not too surprising, given the kind of project this is, but I'm not done yet. I might find something tomorrow."

"Morgan's office and quarters were clean," Gibbs said. "I found her journal, and will read that tonight."

"Interviewed Thomas's team," Tony added, only half paying attention to the conversation. The other half of his attention was still on the man with the odd tattoo-scar. Something was definitely off about him. Now all Tony had to do was figure out what. "They all say it was a routine mission."

"Routine missions don't end in death," Gibbs pointed out.

"Oh!" McGee sat forward. "I almost forgot. Ducky found something in Major Thomas's autopsy report. Not the report, really, but photos -"

"McGee!"

"A pair of holes in the base of the major's brain, tiny holes. Ducky said that a fiberoptic thread inserted into the brain from that location could influence someone's behavior."

"Parasites," Tony murmured.

"What?" Gibbs snapped.

"Parasites. Doc Brown said the team went through quarantine to make sure they didn't bring anything back from their mission," Tony explained. "Like a virus…or parasites."

"Could Major Thomas have somehow been infected by them?" McGee asked.

Tony didn't hear Gibbs' answer. He'd finally realized what was off about the tattoo-scarred man.

 _They wouldn't let Ziva on this base. How the hell did a … whatever tribe he's from get on it?_

An answer formed in his mind, and he turned it this way and that, looking for problems with it. There weren't any - it made perfect sense - and he opened his mouth to share, only to snap it shut before he spoke.

If he were wrong, McGee would never let him hear the end of it. He'd have to confirm his answer before he shared it.

Tony rose to his feet, his dinner forgotten, and scanned the room for the sergeant who'd escorted him to the chow hall.

"You got something, DiNozzo, share it," Gibbs snapped.

"Rule three and rule eight, Boss. Sergeant Lumley!"

"Sir?" Lumley looked up from his dinner.

"Sorry to interrupt your meal, but I need to see Colonel O'Neill."

NCIS - SG1 - NCIS

If asked, Tony wouldn't be able to describe the route to O'Neill's office later. His mind was still too busy sorting through the implications of his conclusion to take note of his surroundings. Thankfully, his distraction happened in a safe place. A similar distraction under other circumstances could cost him his life.

Sergeant Lumley came to a stop before a closed door, and as he knocked, Tony took a long, slow breath and let it out. The action calmed his body, if not his mind.

"C'mon in," came the voice from the other side of the door.

Lumley pushed it open. "Agent DiNozzo to see you, sir."

Tony didn't hear a response, but a moment later, Lumley stepped aside to gesture him into the office.

The door clicked shut behind him even as he instinctively surveyed the office - bookshelves weighted down books on astronomy and physics, as well as military history and tactics; a few odd sculptures; a decided lack of family photos. O'Neill seemed to be married to his job. Tony knew the type.

"What can I do for you, Agent DiNozzo?" O'Neill asked.

"Colonel." Despite the fact that O'Neill practically lounged in the chair behind his desk, Tony straightened his spine like he'd rarely done after leaving Rhode Island Military Academy and spoke before he could talk himself out of it.

"It's aliens, isn't it?"

O'Neill was silent long enough that Tony started squirming internally, doubting his conclusion.

Then O'Neill shook his head. "Guess I owe Morrow fifty bucks. He said you'd figure it out."

Tony's mind reeled as the meaning of the colonel's words sank in. "He did? Then …?"

"Yeah, it's aliens. Sit down before you fall down."

Tony fumbled for one of the chairs facing O'Neill's desk and half-sat, half-collapsed into it while he stared at O'Neill. "It's aliens."

"You figured it out - why are you so surprised?"

"I expected you to deny it."

O'Neill chuckled. "Morrow wanted your team read in completely from the beginning. General Hammond, the base commander, overruled him."

Tony felt his eyebrows rising. "A BC overruled a deputy director?"

"Well - persuaded him otherwise. How'd you figure it out?" O'Neill seemed genuinely curious, and Tony leaned back in his chair, hoping he appeared casually confident.

"Lots of little things," he said. "Lieutenant Calloway seemed surprised when I asked him how Major Thomas behaved on the drive back from Peterson."

A crease appeared in O'Neill's forehead. "Huh?"

"Where else would they have flown out of for their mission?" Tony asked. "It's not like you have an airfield under the mountain… do you?"

"No. Go on."

"You have archaeologists accompanying your teams - which I figured meant you were trying not to destroy any antiquities doing whatever it is you do." Tony shook his head at himself. "But there's another reason to take an archaeologist along - to interpret something you find, something you might be looking for. There's not a lot to look for on this planet that would require this strong a military presence."

"What else?"

"Doc Brown mentioned parasites and quarantine. It makes sense, but parasites are fairly harmless - parasites die if their host dies, after all."

O'Neill considered that before nodding. "I can see how those pieces suggest the outlines of a puzzle. How'd you fill it in?"

"On the way to the chow hall, I saw this guy with a scar-tat on his forehead." Tony drew an oval over his own forehead. "Which made me think - you wouldn't let Officer David on this base, so how the hell did a guy from some tribe from God knows where get on it?"

"Tribe?" O'Neill lifted an eyebrow at him. "Why did you think tribe, not gang?"

"I was a cop for six years before I joined NCIS, and I've kept in touch with local PDs the eight years I've been with them," Tony said. "I don't know all the gang marks, but something like that would stand out."

Tony watched O'Neill's reaction, and decided that when he wanted, the colonel had almost as good a poker face as Gibbs. Tony shrugged.

"Then I remembered what Sherlock Holmes said - once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth."

"Lotta people would say aliens rank among the impossible," O'Neill observed.

"Only if they buy Fermi's paradox instead of Drake's equation," Tony shot back, and bit back a grin at O'Neill's surprise. "Or if they hadn't seen _Men in Black_."

O'Neill chuckled. "Great movie."

"Guessing the aliens you deal with are less friendly?" Tony ventured.

"Depends on the alien," O'Neill countered. "Teal'c - scar-tat guy, he's gonna love that - is a friendly. A race called the Goa'uld are decidedly not friendly. They're also the parasites you were referring to earlier."

"Huh," Tony said. "You wouldn't think something that small could become a problem. An intelligent problem, I mean."

"Not that small," O'Neill said. "More like snakes."

"Garden snake, cobra, or python?"

O'Neill considered that for a moment. "Combination garden snake and cobra. Nasty things."

"Sounds like." For a moment, Tony let himself feel the reality of a universe in which humans weren't the only intelligent life. Then he brought his attention back to the colonel. "So we could be looking for an alien killer."

"Maybe. In Thomas's case, could be probably." Then O'Neill stood. "Let's go meet with your team, and I'll fill you all in."

"Just - you be the one to tell McGee aliens are real," Tony said. "He won't believe me."


	5. Chapter 5

Tim had watched Tony stride off - not running, but certainly hurrying - with the sergeant who had escorted him to lunch. Tony hadn't even glanced back over his shoulder. When he looked at Gibbs, he saw that his boss appeared unconcerned.

"Uh - Boss?" Tim ventured.

"Yeah, McGee?"

"You're just -" Tim heard the accusation in his voice and cut himself off. He started again, this time keeping his tone to merely inquiring. "Why are you just letting him run off like that? Shouldn't he share what he's found?"

"You heard him," Gibbs said. "He doesn't know what he's found, and won't until he follows rules three and eight. Meanwhile, he's not wasting our time with it."

"Huh. I see." Only he didn't - or maybe that was just that he didn't see whatever Tony might have seen, whatever could've possessed the other man to dash off in the middle of dinner.

Speaking of which… A glance at the table confirmed that Tony had barely touched his food, and for a moment Tim debated with himself, finally deciding to go to the service line and ask for a to-go box. Tony was always easier to deal with on a full stomach - both his and Tim's.

Surprisingly, Gibbs lingered over an after-dinner cup of coffee. Tim wasn't sure whether he should go back to work, or whether - _please God_ \- Gibbs might call it a day, given that it was pushing nine p.m. local time, which meant their bodies felt like it was already eleven.

Still, if Gibbs lingered, then he could, too - although there was no way he could drink coffee right now if he wanted to sleep at all tonight. If he'd sleep at all tonight, he corrected, knowing that depended on how the case played out.

Then Gibbs was draining his cup, and Tim figured it was back to work for them. He bussed their meal trays and caught up to Gibbs just as the other man was heading for the door.

Their escort met them there and guided them back to the conference room - where Tony and Colonel O'Neill waited for them.

"Go on, Sergeant," O'Neill told their escort. "I'll escort them to their quarters when we're done."

"Quarters?" Gibbs asked - _asked_ , Tim noted, not _demanded._ This was a side of Gibbs he hadn't seen before, and while it wasn't quite as unnerving as the _nice_ Gibbs he'd seen in the aftermath of Ari Haswari's assault on NCIS, it was still a side Tim hoped never, ever, to see again.

"Don't think you'll be able to concentrate after this," O'Neill said casually and gestured them to take a seat.

Tim did, wondering what _this_ might be. Gibbs, though, just studied Tony.

"Rules three and eight?" he asked.

"Oh, yeah," Tony said. "Not that I expected it to pan out."

O'Neill frowned. "Rules three and eight?"

"Never believe what you're told," Tony said. "Always double-check. And eight is never take anything for granted, or never assume."

"Variations on a theme," O'Neill said.

"Necessary in our line of work," Gibbs countered.

O'Neill nodded. Then he took a breath. "I shouldn't have to remind you of the nature of this base, and this project, nor of the oaths you've taken, nor the punishments awaiting you if you break those."

Gibbs grinned, just a little. "You just did."

"Necessary in this case." O'Neill studied Gibbs and then Tim, and Tim found himself straightening in his seat. Growing up with a career-Navy father, now an admiral, had instilled both a respect for the military in general and a somewhat insouciant attitude toward its requirements. Somehow, O'Neill's scrutiny burned through that attitude.

"You know Tom Morrow as an assistant director of Homeland Security," O'Neill said. "What he really is, is Director of Homeworld Security."

"Homeworld?" Tim repeated, feeling his forehead crease into a frown. "But that makes no sense."

"It implies a danger from other worlds," Gibbs said slowly, carefully.

Tim blinked at him, not sure whether he was more surprised by the suggestion or by the fact that Gibbs had made it.

"We're the first line of defense," O'Neill said, and while he had a streak of sarcastic humor to rival Tony's or Tim's own, in this moment the colonel was absolutely serious, and Tim sat forward.

"You're talking about invasion from another world," Tim blurted. "You're talking about aliens."

"I am."

"But that's not possible," Tim said. "The Fermi Paradox -"

"Fermi was brilliant," O'Neill said. "But in this case he was wrong. Drake's right."

"Do either of them have anything to do with this case?" Gibbs almost snarled.

"Uh - no, Boss. Not really," Tim said before focusing on O'Neill once more. "I want to see one."

He forced himself to ignore Tony's grin in favor of Gibbs' approving grunt.

"Rule three," Gibbs muttered, then he, too, looked up at O'Neill. "Or, as Sagan said, extraordinary claims demand extraordinary evidence."

O'Neill looked between them, then rolled his eyes. He opened the conference room door, leaned into the hallway, flagged down the nearest passing person - Tim was certain O'Neill didn't care whether said person was officer or enlisted - and spoke quietly to them.

"He'll be here in a minute."

Gibbs picked up on the pronoun immediately. "He. So there's only one on base?"

"At the moment," O'Neill said. "He works with my team primarily and offers assistance to others when needed."

"You know I have to ask," Gibbs said.

Tim wondered what Gibbs had to ask, but O'Neill apparently understood the unspoken question. "He's saved my life a handful of times, and I've saved his about as many. He's earned my trust."

"I hope it's not misplaced," was all Gibbs said.

The silence that settled was … not comfortable, exactly, Tim decided. Rather, it was expectantly neutral, or maybe neutrally expectant. He rolled both phrases over in his mind. One of them would make it into his next book.

Thanks to years, decades even, of getting lost in video games or programming, Tim's internal time sense sucked, so he had no idea how long it was before the conference room door opened and a tall, bald man apparently of African-American ancestry strode in.

Tim couldn't help staring at the gold oval raised tattoo on the man's forehead, only tearing his gaze away when the man's eyes landed on him - not judging so much as accepting. Then the newcomer looked at the colonel.

"You wished to see me, O'Neill?" The man's voice was rich and resonant, though he clipped his words in a way that Tim had never heard before, not even from Gibbs.

"Yeah, Teal'c, c'mon in."

The man called Teal'c stepped into the room, allowing the door to close behind him.

"This is Teal'c," O'Neill said. "Teal'c - the NCIS investigators looking into Majors Thomas and Morgan's deaths."

"You said that I am not to associate with them," Teal'c said.

"Yeah, I did," O'Neill admitted. "And then this one -" he jerked a thumb toward Tony -"figured out you're an alien."

Teal'c took two large steps and towered over Tony. "How? We barely saw each other in the corridor earlier."

Tim wanted to back away from the man - alien, whatever he was - and he was across the conference table from him. Tim could only assume Tony felt the same, but as always, Tony covered whatever he really felt with humor and misdirection.

"To be fair, seeing you was only the last piece of the puzzle," Tony said. "There were a lot of others already in place."

Teal'c scowled, and Tony shrugged. "I already went through every step for Colonel O'Neill. How about I just say there's something off about you - you're not like the rest of us, and I don't mean that in a bad way. But when I tried to figure out what it is that was off about you, I got to alien."

"How?" Teal'c demanded again, and Tim could only hope Tony had all his affairs in order. The other man - the alien - looked ready to kill.

"He's good," Gibbs' voice cut into the silence. "Best investigator I've ever worked with."

Tim blinked at the compliment - less at who it was directed toward than that it was given at all. But Teal'c still looked ready to kill, and if anyone was going to kill Tony, it would be Tim. Or maybe Gibbs, he added silently. In either case, no alien was going to kill Tony on his watch.

"So, if you're an alien," Tim said, "what planet are you from?"

Teal'c straightened and regarded him impassively. "Chulak."

"But you look human," Tim continued, remembering rules three and eight, even if Tony had theoretically already followed them.

"The peoples of many planets look like Earthlings," Teal'c replied.

"It's a long story," O'Neill put in. "But the short version is, a lot of ancient humans were taken to other planets a long time ago."

"Funny," Tim mused. "I always thought aliens, if they existed, wouldn't look anything like us."

"Count yourself lucky, Probie," Tony said. "The colonel told me about other aliens that look more like snakes - nasty snakes."

Tim felt his eyes widening. "Them, I'd have to see to believe."

"Very well." Teal'c turned to face him and before Tim could speak, Teal'c tugged the hem of the black T-shirt he wore up almost to his pectoral muscles, revealing an X-shaped incision or slit in his abdomen.

 _What the…?_

Tim could only stare, transfixed, as the slit opened like flower petals, and from the pouch concealed beneath it emerged a pinkish snake - or something like a snake, Tim corrected himself. It had two lips, that also opened in an X-shape, revealing long spiking tendrils emerging from its mouth.

"Jesus," Tony whispered. "You said snakes, but I never expected _that_."

"Neither did I," O'Neill's quipped dryly. "That's more than they needed, Teal'c."

"But exactly what they requested." Still, the snake-like thing retreated into its … Tim could only call it a womb.

"Did you kill Major Thomas or Major Morgan?" Gibbs' question pierced the silence like a bullet from one of the man's sniper rifles.

"No." Teal'c's voice never wavered, and neither did his gaze. Tim noted these details thanks to his training, but otherwise he was focused on one thing.

"Those … snakes," he said.

"Goa'uld," O'Neill supplied.

"Yeah, those," Tim agreed absently. "Those … fangs? Whatever you call them. They look small."

"Less than a millimeter in diameter," O'Neill agreed.

Tim swallowed hard, but met O'Neill's gaze. "Ducky - our medical examiner - found two holes, less than a millimeter in diameter each, in Major Thomas's brain. Could one of those gah-olds have made them, somehow?"

Just thinking of the possibility, let alone speaking the words, had bile rising in Tim's throat. To have something like that _inside him_ , poking into his brain… Tim shied away from the thought.

O'Neill didn't, though, not if his serious expression were anything to go by. O'Neill turned to Teal'c. "Could one have gotten past the screening?"

Teal'c didn't hesitate. "Any security measures can be breached."

"Gotta ask the obvious," Gibbs said. "You sure the one inside you didn't kill them?"

"It is not possible," Teal'c said. "Only a mature goa-uld is capable of invading a body. The one I carry is not yet mature."

"But what about Major Morgan?" Tony asked. "Those things don't look like they can wield a knife."

"They cannot," Teal'c agreed. "But their host bodies can."

"Host… bodies." Tim barely recognized his own voice, as weak as it was. But at least he wasn't stuttering.

Tony looked between Gibbs and O'Neill. "What was Major Morgan working on when she died? Anything that might have threatened the thing inside Thomas?"

"I didn't see any notes in her case files," Gibbs said. "Was planning to read her journal tonight."

"She hadn't come to me with anything," O'Neill added. "But she might not've - not if she wasn't sure, or not if she suspected I'd been compromised."

"Are you sure you're not?" Tony asked, and Tim wondered that his teammate seemed to be taking this in stride. Then again, Tony had figured out that they were dealing with aliens in the first place - _how?_ Tim resolved to ask him when this was over.

"Pretty sure," O'Neill replied. "Ever since one of them took over Kawalsky, we've had mandatory screenings when we return from missions. Besides, I still feel like me, I still talk like me. When one of them takes you over …"

"The host is lost," Teal'c finished.

"Rule thirty-five," Gibbs said, and Tim racked his brain, trying to remember whether he'd ever heard that rule before.

"Always watch the watchers," Tony supplied, and Tim breathed a silent sigh of relief. At least he hadn't forgotten one of the rules.

"In context," Tony continued, "when was the last time the screeners were screened?"

Whatever answer O'Neill might have made was cut off by the shriek of an alarm klaxon blaring through the loudspeakers.

"Alert - inbound traveler." The voice that followed the klaxon sounded remarkably calm. "Repeat - inbound trav-"

The voice cut off abruptly, and O'Neill and Teal'c barely exchanged a glance before they were tearing out of the room. Almost before Tim realized what was going on, Gibbs and Tony were following them.

Another second later, Tim's body caught up with his brain, and he pounded after his boss and SFA.


	6. Chapter 6

Later, Jethro wouldn't be able to say what exactly made him follow O'Neill and Teal'c from the conference room. Was it simply the reaction of a career Marine? Was it loyalty to a man he barely knew but already recognized as a leader he could follow? Or was it merely the instinctive drive to protect his home?

Whatever the reason, he pounded down the corridor, O'Neill and Teal'c just ahead of him, DiNozzo beside him, McGee hurrying to catch up.

O'Neill and Teal'c paused at what had to be an armory, given that a stream of troops filed by, each accepting weapons and ammo before continuing on their way.

O'Neill barked an order - Jethro thought he said, "Staves," but he wasn't certain - and then frowned at him. "What?"

"Marine sniper," Jethro told him. "How can we help?"

"You can't," O'Neill replied shortly. "Not this time - I've got good men here, and I'm not compromising them by throwing unknowns in the middle of a war zone."

"War zone?" Tony asked.

"Invasion," O'Neill shot back, accepting a very high-tech looking spear-like object from the armorer. Beside him, Teal'c accepted an identical one - the staves he'd referred to, no doubt. "At the least a scout force."

Jethro assumed what O'Neill and Teal'c did next was a weapons check, then O'Neill met his gaze once more. "Davies should have the gate closed soon. Meantime, stay out of our way."

Jethro's lips tightened, but he only nodded. He understood O'Neill's point, but he was, when all was said and done, a man of action - specifically, this kind of action: fighting for his life and the lives of those he cared for, and those he'd never meet but who nonetheless didn't deserve war raining down on them.

"Ready?" O'Neill asked Teal'c.

Before Teal'c could reply, a blonde woman in a captain's full battle kit with a rifle slung over her arm rushed toward them. "Colonel! Davies is down."

"Dammit," O'Neill swore. "Then get -"

"Down, too," the woman replied. "All of the techs are down."

"Efficient," Teal'c observed - and while that observation was very likely true, even Jethro knew its timing was lousy.

"Go on, Teal'c - I'll catch up," O'Neill ordered. Teal'c nodded, and thundered down the corridor, shoving regular troops out of his way as needed."

"Make a hole!" DiNozzo yelled, and the men parted to allow Teal'c better passage.

"What about you, Carter?" O'Neill asked.

The woman shook her head. "I understand the language, but I'm no good with computers."

"I am," McGee said, and Jethro felt a surge of pride in the competent agent standing beside him. The McGee he'd first met would've run screaming from this situation, but now he stepped up when he was needed.

Then he was watching O'Neill as the man's expression shifted.

 _Probably considering various scenarios,_ Jethro mused. _I would be. Hell - I am._

"All right," O'Neill said finally. "McGee - you go with Carter, find a way to close the gate. Gibbs, DiNozzo - hold that room."

"Yes, sir," Gibbs snapped, even as he wondered, _What room?_

Then he was accepting a rifle - a Mk 14 Enhanced Battle Rifle - and following Captain Carter and McGee along the same path Teal'c had taken.

Carter bellowed, "Coming through," in a tone that would've made Jethro's former drill sergeant proud, and less than a minute later, the four of them spilled into a room that could have been the bridge of a spaceship on some old television show. DiNozzo helped Carter secure the door behind them.

Jethro assessed the situation in a glance. Whoever sat at the bank of computers faced a glass window that looked out over … Jethro blinked at the combat zone below them.

A circular sculpture of some kind dominated the far end of the room, and emerging from its watery center were figures in armor that reminded Jethro of the part-human part-animal gods of Egypt Kelly had showed him once.

They - the enemy - marched through the gate and down the ramp leading up to it, carrying lances like the ones O'Neill and Teal'c had retrieved. Short bursts of light emitted from the pointed ends of the lances.

"Lasers?" DiNozzo muttered. "Terrific."

"Forget them," Carter ordered. "Our job is here. McGee - have a seat."

McGee did as ordered, and Jethro tuned out Carter's explanation - the computers were her and McGee's job. His and DiNozzo's job was to protect them while they did whatever they did.

Another look through the window made him smile. He had high ground and remarkably close range. Time for him to go to work.

"DiNozzo - tell me if the door's breached," he ordered.

"On it," came the immediate acknowledgment.

Jethro surveyed the room. Not a lot of cover presented itself, so he went to the far corner and shot a hole in the bottom corner of the window.

"A little warning next time," DiNozzo yelled, only it sounded distant and hollow thanks to the ringing in his ears from the rat-a-tat of shots in a confined space.

Jethro ignored him, instead crouching so he could sight down the rifle through the hole he'd made.

Even from here, he could tell that although the bolts from the lances O'Neill and Teal'c wielded were effective against the invaders' armor, the rifles weren't.

He allowed himself a small smile. Any armor had weak points, and from this vantage, he could spot them all. Fortunately, these invaders - Jethro's mind refused even to form the word _aliens_ \- were human, and that meant their armor had the same weak points he was already familiar with, especially the shoulders and the knees. And - _oh, good_ \- on top of that, it looked like their sides and upper legs were less armored than their head, chests, and hips.

 _Target-rich environment._

Jethro lined up on the invader nearest him and squeezed the trigger. The man's right thigh collapsed beneath him, and he fell to the ground. Jethro saw him scrambling to staunch the flow of blood for an instant before moving on to his next target.

NCIS - SG1 - NCIS

The moment Gibbs slipped past him toward the far corner of the room, Tony positioned himself between McGee and Carter and the door. His assignment was clear - to hold the door against whatever might come through it that wasn't someone he knew.

His first step was to survey the room below him for the likelihood of any alien invaders breaching the first line of defense.

 _Not likely_. Between Gibbs' targeted shots, O'Neill and Teal'c's lasers - _lasers! Cool!_ \- and the blanket of fire laid down by the regular troops, the invaders were almost fully stopped at the bottleneck.

Just his luck - an alien invasion, and Tony would never get to fire even one shot in defense of his home. Then again, even if he did, he couldn't tell anyone about it, so maybe the whole situation was a wash.

With the room secure for the moment, Tony risked a glance over his shoulder toward McGee and Carter.

They spoke quietly, clipped words that Tony could barely make out and had less chance of understanding. Still, he thought that was a good sign - at least when doctors were talking, things were going well. He had to hope the same was true for geeks.

A high-pitched whine cut off any further musing on the topic, and he turned to face the window separating them and the ... staging room? It was as good a name as any … Just in time to watch the window shatter. Beyond it, O'Neill and the troops, even Teal'c, staggered backward, many collapsing with their hands clapped to their ears.

In the next handful of seconds, three things happened:

The invading party surged forward, taking advantage of the chaos following that incapacitating whine.

The defenders fell back, some even staggering through the open door to the corridor.

And another double handful of invaders stepped through the gate.

"Can you close the gate?" Tony demanded.

"Trying," Carter snapped back.

In a slice of space between the invaders' bodies, Tony caught a glimpse of a device he hadn't seen before - perhaps hip height, with a dome on top a square body. One of the invaders stood behind it, and Tony thought he might be making some kind of adjustment to it. Maybe he was charging it to emit that whine again?

Even if it wasn't the source of the whine that had turned the tide against the defenders briefly, Tony decided that a device that size boded no good.

"Gibbs," he called. "Can you get that boxlike thing in the center?"

Gibbs squeezed off several shots in quick succession before saying, "Negative. Not high enough ground, and too many bodies between me and it."

Tony glanced back into the staging room. The press of bodies around the device had just gotten thicker in the last couple of seconds as more invaders streamed through the portal.

Another glance told him that most of the defenders were retreating into the corridor. Then O'Neill's order chilled him.

"Seal the room. Set auto-destruct."

That O'Neill was still inside the staging room only underscored the seriousness of the situation.

"Dammit," Carter muttered, and reached for a control on the panel.

"Hey, hey -" Tony slapped a hand on her wrist. "What're you doing?"

"You heard him," she shot back. "We can't let them get past this base, and with that machine they've got, it won't be long before we're all down. We'll collapse the base on top of us and stop the invasion right here."

Tony's thoughts whirled. He knew she was right - the invasion had to be stopped here - but he didn't want to die. There had to be another solution….

He never expected to find it while he was staring down at Carter - at her chest, or more specifically, the bandolier of fragmentation grenades slung diagonally across it.

"Sorry," he muttered, and, letting his rifle drop to dangle on its strap, snatched two grenades from her bandolier.

"What the-!"

Tony ignored her cry and fell back against the wall opposite the window. So far, the invaders hadn't started firing at them, which meant he had a few seconds.

 _Plenty of time._

He scanned the room below - at least his target this time was stationary, not a receiver running a complicated pattern.

With a silent prayer, Tony popped the pin from the grenade. _Four seconds_. He took a step forward and threw it through the window. _Three seconds._ The second grenade followed after as quickly as he could line up the throw. _Two seconds._

"Fire in the hole!" Gibbs yelled. _One second._

Tony dove for cover, taking Carter with him. McGee dove out of his chair, and Gibbs fell on top of him.

 _Zero._

Two explosions rocked the room, and Tony thought his ears wouldn't stop ringing for a week. He shoved himself to his feet, readying the rifle in his hands as he looked over the window into the staging room.

To his left, Gibbs did the same, sweeping his rifle in a slow arc as he searched the wreckage of bodies and steel for any enemy that might still be alive. There was no movement from the pile, and Tony aimed at the portal that was somehow still standing and, worse, still open to allow more invaders to come through.

"Got it!" McGee's voice sounded tinny through ears damaged by too many bangs and explosions in a confined space. Before Tony could ask what, exactly, he'd got, the portal collapsed and a shield swirled closed to cover it.

Only then did Tony turn to offer a hand up to Captain Carter, but she was already on her feet, surveying first the remains of the staging room where many of the defenders - including Colonel O'Neill and Teal'c - climbed slowly to their feet.

Thankfully, the defenders who'd heard Gibbs' shout had hit the deck, and the invaders' armor, and bodies, had shielded them from the worst of the grenades' damage. Still, Tony knew he'd lose sleep over those defenders who hadn't survived the blasts. But that was for tonight, and tomorrow. For now, he accepted Carter's hug.

"Good arm," she said, and Tony could only nod an acknowledgment. In other circumstances, he'd make some comment about being the starting quarterback for Ohio State, but this moment was not a time for humor.

Gibbs' hand landed on his shoulder, and he met his boss' gaze.

"Atta boy, DiNozzo," he said, and again Tony could only nod.

Then Gibbs turned to Carter. "That thing secure?"

Carter nodded. "The iris is pure titanium. So far, it's proven to be impenetrable."

"But - if they're still coming through," McGee began. "They'll be -"

"Bugs on a windshield." The voice came from Tony's right, and he whirled to face this possible new threat, only to lower his rifle when he recognized O'Neill.

He tried not to wince when he saw the handful of lacerations down O'Neill's left side. They could only have come from the grenades he'd thrown, and Tony suspected he'd be seeing them in nightmares to come.

"So we won?" Gibbs asked.

"For now," O'Neill replied. Then he grinned, just a little. "Guess you could help, after all."


	7. Chapter 7

It was two more days before they were allowed to leave the mountain - two days of cleanup and debriefing and a hurried memorial for those who'd fallen defending their home planet.

It was, Tony reflected, both more exciting and more boring than the movies made it seem. They'd finished up the case files on Majors Thomas and Morgan, handed over their notes to O'Neill, and met with the base commander, General Hammond, for debriefing and, surprisingly, a word of thanks for their assistance in defending the attack.

"I'll see that commendations are put into each of your files," Hammond said. "It's poor thanks for your assistance, but it's the best that I can do."

Then they were on a transport back to DC as though nothing untoward had happened.

And, Tony thought, as far as anyone else was concerned, nothing untoward _had_ happened. Colonel Kennedy reminded them of that quite forcefully at a second debriefing when they landed at Anacostia-Bolling, and Tony bit back his observation that even if they told anyone what they'd done, no one would believe them.

Then they were back at work. Unsurprisingly, Ziva asked about what had happened, undeterred by reminders of _need to know_ and _orders from above._ When she loomed over McGee, Tony snapped.

" _Officer_ David. Put yourself in his place. If someone questioned you about a classified operation, what would you do?"

Ziva blinked at his tone - one Tony rarely used because he'd hated it so much when his instructors at Remington Military Academy used it on him. Then his words sank in, and she turned to McGee.

"My apologies, McGee. I will not ask again."

After that, things returned to normal - at least mostly normal, Tony amended. The banter they normally shared seemed stilted and formal at first, but as the nightmares of combat and snakes emerging, _Alien_ -like, from the bodies of people they knew faded, the jokes returned, however slowly.

Two weeks after their return, they sat finishing reports on a smuggling case they'd just closed when the phone on Gibbs' desk rang. Tony instinctively reached for the drawer where he stowed his gun. But then Gibbs was hanging up and _not_ rising from his chair, _not_ saying, "Grab your gear," and Tony sat back in his seat, watching his boss for any hint of what might be going on.

Then the elevator dinged and Tony looked up to see a figure in Air Force ABUs emerge from it. He recognized Colonel O'Neill immediately, and rose to meet him.

"Let me guess," O'Neill said as he approached, casting a critical eye on the room. "Lowest bid on the paint contract?"

Tony grinned, and for the first time since he'd returned from Colorado, it felt natural. "Either that or somebody was tripping on controlled substances."

"What is an Air Force colonel doing in a Navy facility?" Ziva's question cut through the bullpen.

"Ziva -" Gibbs began, but O'Neill just grinned at her.

"Catching up," he said.

Her eyes narrowed and her lips tightened. She nodded once, then returned her attention to her computer.

"Someplace we can talk?" O'Neill asked.

"Conference room," Gibbs replied and led the way, gathering McGee and Tony with a glance.

When the conference room door was closed securely behind them, O'Neill fixed each of them in turn with a steady, assessing gaze.

"You guys holding up okay?"

"Fine," Tony said, while Gibbs grunted and McGee nodded.

"Uh-huh." O'Neill's tone conveyed his disbelief. "And I'm the Queen of England."

"Look," McGee said, and there was only a hint of a stammer in his voice. "I can't speak for Gibbs or Tony, but - yeah, it was intense. But it's also kind of what we do, you know? Protect the innocent, and all that."

"Against all enemies, domestic and foreign," Gibbs put in. "Though _foreign_ has a wider meaning now."

O'Neill studied each of them in turn before finally turning his gaze on Tony. "You?"

Tony shrugged. "I did what I had to do, and I know that. Doesn't mean I don't regret the friendly fire."

"They died knowing their deaths prevented more," O'Neill said. "It's enough."

"It will be," Tony corrected, and after a moment O'Neill nodded.

"There a problem at the mountain?" Gibbs asked when the silence lingered.

"No," O'Neill replied. "Just some reports are best given in person, so here I am. Morrow sends his regards - and his thanks. And, because he's Morrow, he also sends these."

He shoved a hand into his pocket and when he withdrew it, Tony could see he held something concealed in his hand.

"C'mon," O'Neill urged, holding out his hand as though to drop something he held.

One by one, Gibbs, then McGee, and finally Tony extended their hands, palm up, to receive whatever he might offer them.

Then Tony was looking at the challenge coin lying in his palm, the words _Secretary of Homeland Security_ engraved on the outside surrounding the seal of the office. When he looked up, Gibbs was pocketing his, and McGee was examining his with the focus of a laser.

"Morrow wanted to get SecDef coins for you," O'Neill observed. "But I told him that would just be pretentious."

Gibbs snorted, and Tony chuckled - the first honest laugh he'd had since he returned from Colorado. Then a question occurred to him.

"How does the challenge work," he asked, "when there's a tie? Say the three of us were challenging each other - we all have the same rank coin. Who gets drinks bought for him?"

"It goes down the line - whoever has the next highest coin," O'Neill replied.

"Oh," McGee said, perking up. "I have an admiral."

He dug in his pocket and produced the coin, showing it around before grinning at Tony. "Guess you're buying me a drink."

"Doesn't count if your father gave it to you, McGee," O'Neill observed mildly. McGee frowned at him, but Tony just shrugged.

"Doesn't matter," he said. "Even if it weren't his dad, I win."

"Tony." McGee's tone held that lecturing note Tony had come to loathe. "This is an admiral's coin."

"Yeah, McObvious - I can see that." In turn, Tony dug into his pocket, pulling out a handful of coins and picking through them. "Let me see - captain, captain …"

"You have two captain's coins?" Gibbs asked, appearing mildly curious.

"Yeah," Tony replied absently. Who knew he carried so many coins - drinking and otherwise - in his pocket these days? "Captains of the _Gipper_ and the _Seahawk_ each gave me one when I was afloat."

"You called a Navy ship the _Gipper_?" O'Neill's tone conveyed both disbelief and scorn - typical, Tony thought, of the rivalry between branches of the service.

"We have a ship nicknamed the _Gipper_ \- the U.S.S. _Ronald Reagan_. And … we have a winner." Tony added as he found the coin he'd been looking for.

He extended his palm to let the others examine the coin resting on it.

O'Neill whistled. "Secretary of the Navy. Impressive."

McGee stared at the coin in Tony's palm, dumbfounded, before turning an angry glare on Tony. "How'd you get _SecNav's_ coin?"

"Domino," Tony replied shortly. Those were memories he didn't want to relive. "And before you say you were there, too - you were in the van, not on the front line."

O'Neill clapped him on the shoulder. "Guess I'm buying the first round tonight."

Tony blinked. "Tonight?"

"I'm only in town tonight," O'Neill said, "and I usually buy guys who fought under me a round. Unless you're busy?"

"No," Tony began, but cut himself off and looked at Gibbs for confirmation.

Gibbs' mouth twitched. "Second round's on me."

* * END * *

I hope you enjoyed this first (yes, first) look at what I think might happen if the NCIS crew met the SG-1 crew. The sequel will be up when it's done. Thanks for coming along for the ride!


	8. Chapter 8

The sequel to this story, titled "PCS," is now up. I hope you enjoy it!


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